Stress field in the Vanuatu (New Hebrides) Wadati–Benioff zone inferred by inversion of earthquake focal mechanisms: evidence for systematic lateral and vertical variations of principal stresses

Abstract

We evaluate the stress tensor directions in several depth ranges along the Vanuatu Wadati–Benioff zone by inversion of 550 earthquake Harvard CMT solutions. Our results show that although the strike of the Vanuatu slab varies by over 70 °C, the maximum compressive stress σ1 is, nearly horizontal and slab normal all along the arc in the depth range 0–180 km. The minimum compressive stress σ3 is down dip in the top 60 km and shows a clear tendency to vary systematically from down dip to strike aligned in the depth range 61–180 km. At depths below 180 km σ1 rotates clockwise at about 30° in respect to the slab normal, while σ3 is down dip. The variations of σ3 direction below 60 km indicate that horizontal stretching of the slab modifies the slab pull force. The universal direction of σ1 in the range 0–180 km implies that one of the dominant forces at Vanuatu is nearly horizontal and slab normal. In the top 60 km it is most likely related to the plate convergence. The only plausible force of this type at greater depth is the sea anchor force: the viscous resistance to the face-wise motion of the slab relative to the mantle. The change of σ3 direction from strike aligned to down-dip tension at depth greater than 180 km suggests that the slab pull force may become more prominent with depth.